I think 6mm perlon is perfectly adequate for the job, and 7mm can be unwieldy to tie three legs of a cordolette together or to have hanging on your harness.
Just to point out, Gene's calculations above assume two anchor points perfectly above the power point, with zero angulation, which is essentially never the case. The forces on each leg will be considerably greater as these points move apart.
It's generally a good idea to have a healthy safety margin between the material breaking strength and the maximum load you anticipate, rather than simply counting on the rated strength.
I like to use a general rule of thumb that any knot in a cord reduces the strength by 1/3rd.